Method of manufacturing a ductile superconductive material

ABSTRACT

A ductile superconductive material and method for manufacturing same, which basically may utilize any porous metal or its hydride with a melting point substantially higher than that of the infiltrating metal that will form a superconductive compound when reacted with the infiltrating metal. The superconductive material is made from a porous strip or tape of niobium or vanadium for example, infiltrated with tin, aluminum, antimony, antiomony, or gallium, for example, and treated in a manner so as to contain interconnecting filaments of the superconducting phase Nb3Sn, Nb3A1, Nb3(A1,Ge), Nb3Sb, or V3Ga, for example. The novel manufacturing process, which is relatively simple and inexpensive provides a method of producing a new and useful end product capable of a broad superconductive range due to the amount of reaction of the infiltrated material with the porous material. The novel process may be utilized in either batch or continuous operational applications.

United States Patent 1191 Pickus et a1.

' 1111 3,815,224 [451 June l l, 1974 METHOD OF MANUFACTURING A DUCTILESUPERCONDUCTIVE MATERIAL [75] Inventors: Milton R. Pickus; Earl R.Parker,

both of Oakland; Victor F. Zackay, Berkeley, all of Calif.

[73] Assignee: The United States of American as represented by theUnited States Atomic Energy Commission, Washington, DC.

[22] Filed: June 8, 1971 [21] Appl. No.: 151,111

[52] US. Cl 29/599, 29/182.1, 29/191.2, 29/197, 29/198, 29/420.5,174/126 CP, 7 174/D1G. 6 [51] Int. Cl. H0lv 11/00, B22f 3/24 [58] Fieldof Search 29/599, 420, 420.5, 182.1, 29/192 R, 194, 197, 198, 183.5,191, 191.2, 191.4, 191.6; 174/126 CP, DIG. 6

[56] References Cited UNITED STATES PATENTS 2,671,953 3/1954 Balke29/42015 ux 3,069,757 12/1962 -Beggs et a1 29/182 1 3,196,532 7/1965Swartz et a1. 29/420 3,214,249 10/1965. .Bean'et al 29/198 X 3,301,643l/1 967 Cannon et a1 29/192 R 3.317.286 5/1967 DeSorbo 29/194 x3.341.307 9/1967 Tarr et a1 29/182.1 3.352.007 11/1967 Charles 29/599Primary ExaminerC har1es W. Lanham Assistant Examiner-D. C. Reiley, I11Attorney, Agent, or Firm-Roland A. Anderson ABSTRACT 1 The novelmanufacturing process, which is relatively simple and inexpensiveprovides a method of producing a new and useful end product capable of abroad superconductive range due to the amount of reaction of theinfiltrated material with the porous material. The novel process may beutilized in either batch or continuous operational applications.

13 Claims, 4 Drawing Figures METHOD OF MANUFACTURING A DUCTILESUPERCONDUCTIVE MATERIAL BACKGROUND OF THE INVENTION An electromagnetwith the exciting current carried by coils of a superconducting materialwould function with little or no power expenditure except that requiredto maintain the necessary low temperature. The problem in theconstruction of such superconducting magnets of appreciable size hasbeen the difficulty of producing wire with satisfactory physical andmechanical properties at a reasonable cost. In recent years, certainmetallic compounds (alloys) of niobium, e.g., with tin, titanium, orzirconium, have been developed for fabrication into superconductingwires. While the prior art efforts have resulted in substantiallyimproving the state of the art of superconductive materials, the priorprocesses have been complicated and expensive, thus illustrating a needin this field for an effective superconductive material that can bemanufactured by a relatively simple and inexpensive process.

SUMMARY OF THE INVENTION:

The present inventionprovides a ductile and effective superconductivematerial that can be produced relatively simply and inexpensively byeither batch or continuous production. Basically the invention involvesthe forming of a porous strip or tape from niobium or vanadium powder,for example, sintering thestrip, infiltrating tin, aluminum, germanium,antimony, or gallium, for example, into the pores of the niobium orvanadium strip, and diffusion heating treating the infiltrated stripforming interconnecting filaments of the superconducting phase Nb Sn, NbAl, Nb (Al,Ge), Nb Sb or V Ga,

for example, throughout the strip. The amount of the superconductingphase is varied by controlling the time and temperature of the heattreatment. Also, the novel process may include reduction of theinfiltrated strip,

such as by rolling, prior to the diffusion heat treatment which has asignificant effect on the conversion of the infiltrate to thesuperconducting phase during the subsequent heat treatment thereof.Hydrides of the metals, e.g., niobium hydride can be used in place ofthe element. The hydrides decompose to form the porous metal during thesintering process.

Therefore, it is an object of the invention to provide a superconductivematerial and method for manufacturing same.

A further object of the invention is to provide a ductilesuperconducting material containing interconnected filaments of asuperconducting phase throuhout the material.

Another object of the invention is to provide aproces's formanufacturing a superconductive material containing interconnectedfilaments of Nb Sn, Nb Al, Nb (Al,Ge), Nb Sb or V Ga.

Another object of the invention is to provide a method of manufacturinga superconducting material which includes forming a strip of porousniobium from niobium powder, infiltrating the porous strip with tin,aluminum, aluminum-germanium, or antimony, and selectively treating theinfiltrated niobium strip to form interconnecting filaments of thesuperconducting phase Nb Sn, Nb (Al,Ge), or Nb Sb throughout the strip.

Other objects of the invention will become readily apparent from thefollowing description and accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic illustration ofan apparatus for carrying out the operational sequence in a continuousmethod for making superconductive material in accordance with theinvention; and

FIGS. 2-4, illustrate typical structures of the niobium-tin (Nb Sn)embodiment of the inventive superconductive material in various stagesof processing in accordance with the invention.

DESCRIPTION OF THE INVENTION While the following description is directedprimarily to the niobium-tin (Nb Sn) embodiment for purposes ofillustration, it is not intended that the greater detailed descriptionof this embodiment limit the invention to this specific embodiment inthat, as pointed out above and as set forth in greater detailhereinbelow, that the invention can be carried out with othersuperconductive compound such as niobium-aluminumgermanium, Nb (Al,Ge);niobium-aluminum, Nb Al; niobium-antimony, Nb Sb; and vanadium-gallium,V Ga. While efforts conducted thus far to verify the invention have beendirected to the above mentioned compounds, it is currently believed thatthe porous base metal of the strip may be any porous metal with amelting point substantially higher than thatof the infiltrating metalthat will form a superconductive compound when reacted with theinfiltrating metal.

Referring now to FIG. 1 wherein an embodiment of an apparatus isillustrated schematically for carrying out the operational sequence formanufacturing the niobium-tin (Nb Sn) embodiment of the superconductingmaterial as partially illustrated in FIGS. 2-4. While the illustratedprocess is of the continuous type, it may be modified for batch typeproduction if desired.

A hopper 10 containing finely powdered niobium 11,

for example, of particle sizes ranging from l00 mesh to 400 mesh, ismounted above a compacting roller mechanism indicated at 12 which, whenrotated in the direction indicated by the arrows, produces a green stripor tape 13 of porous niobium, the interconnected pores being indicatedafter filling with tin at 13' in FIG. 2. For example, with thecompacting roller mechanism 12 having 2 inch diameter rolls and a rollgap of 0.012 inch, a green strip.l3 having a thickness of z 0.015 inchis produced. The green strip 13 is passed through a sintering furnace 14having, for example, either vacuum or an inert gas atmosphere, atemperature in "the range of 1,950-2,250 C, for a time of about 3minutes, forexample, for a thin strip. The sintered strip indicated at13-1 coming out of the furnace 14 has interconnected pores and aporosity which can be controlled over a considerable range. By way ofspecific example: Density 6.71 GMS/cm 78.3 percent of theoreticaldensity E 21.7 percent porosity. Sintered strip 13-1 is passed through amolten tin bath 15 having direction changing rollers 16 and 17 aroundwhich the strip 13 passes, where the porous niobium is infiltrated(pores filled) with tin producing a tin infiltrated strip indicated at13-2 (see FIG. 2), the tin filled pores being indicated by legend. Forexample, the molten tin bath temperature is in the 500l,0O0C range, andthe immersion time of the strip 13-1 in bath 15 is in the range of a fewseconds to several minutes. As tin infiltrated strip 13-2 emerges frombath 15 it passes over a direction changing roller 18 and through athickness reduction rolling mechanism generally indicated at 19 whereina cold reduction of the tin infiltrated strip indicated at 13-3 isaccomplished due to the strip being ductile (see FIG. 3), theinterconnected tin filaments shown by legend in FIG. 3. Reductions inthickness of the order of 75 percent, for example, presents no problem.However the cold reduction operation is optional depending on thedesired thickness of the strip and the heat treating of the strip asdescribed hereinafter. The thus rolled tin infiltrated strip 13-3 isthen passed through a diffusion heat treating furnace 20 wherein the tinis converted to Nb Sn (See FIG. 4), and the converted strip nowindicated at 13-4 is rolled or reeled on a take-up spool apparatus 21 orother suitable collecting mechanism. The diffusion temperature of thestrip in the furnace 20 may be up to about 1,100C with a preferred rangeof 925C to 1,075C, with a time at that temperature being from less thanl minute to several hours. As pointed out above, the desiredsuperconducting phase is Nb Sn. By controlling the amount of priordeformation, the time and temperature of heat treatment, part or all ofthe infiltrated tin may be converted IO Nb3sn.

If desired, the diffusion heat treatment of-the tin infiltrated strip13-3 may be accomplished by passing the strip through a bath or moltentin instead of furnace 20.

Also, if desired the diffusion heat treating operation can be carriedout as a separate process. The infiltrated strip 13-3 can be coileddirectly on the reel 21, and subsequently heated in an inert atmosphereor vacuum to produce the superconductive phase as above described. Thiswould be a desirable modification of the above described process formaterials that must be heated for long periods of time at lowtemperatures to produce the superconductive phase. An example of this isV Al.

By way of example, with no prior cold reduction of the tin-infiltratedniobium strip, a heat treatment of 2 hours at 975C (ductility: nil)results in appreciable amounts of unreacted tin; while with a priorreduction in thickness of 75 percent, a heat treatment for only 1 minuteat 1,000C (ductility: can be formed around a if; inch diameter mandrel)results in conversion of a major portion of tin to Nb Sn.

To illustrate the advantages of the inventive superconductive material,current density tests have been conducted to determine the currentcarrying capacity of specific strips of the materiaLHowever, it shouldbe noted that the current density will vary with different materials andprocess variables such as the porosity of the strip (which determinesthe maximum amount of infiltrate that can be infiltrated into thestrip), the amount of reduction of the infiltrated strip, and the heatdiffusion time and temperature which determine the amount of reactionbetween the porous strip and the infiltrate, and thus determines thecritical current required to drive the material from a superconductivestate to a normal state. By way of example only, niobium powder of 270mesh was rolled into a strip, sintered, infiltrated with tin, reducedabout percent forming a strip having a cross-section of 0.004 inch by0.055 inch, and diffusion heat treated in accordance with the invention.The thus formed strip of Nb Sn was wound on a mandrel and tested forcritical current density by placing the coil in a variable magneticfield and varying the amount of current passed through the coil. Thetests were conducted in magnetic field settings of 15KG, 20KG, and SOKG.To determine the amount of current flow through the coil required todrive the coil normal voltage readings across the coil were taken. Adetectable voltage reading indicated the start of the transition to thenormal state; For example, with the coil as above described, the testsshowed the starting of the transition to normal as being 8.7 X 10, 7.0 X10 and 3.1 X 10 amps. per sq. cm. for the 15, 20 and SOKG magneticfields respectively. The cross-section utilized to determine the aboveamps/sq. cm. values is the cross section of the ribbon or strip. In thisspecific test the volume fraction of Nb Sn in the strip was estimated tobe about 7 percent. Accordingly, it is readily apparent that byincreasing the volume fraction of the Nb Sn by the processing variablesindicated above, the current carrying capacity of the strip would besubstantially increased.

After the completion of the above current density test the strip wasreverse wound on the mandrel, and retested with the result that therewas no substantial reduction in the current carrying capacity. Thisclearly verifies the ductility of the inventive superconductivematerial.

As pointed out above, the inventive superconductive material is notlimited to the niobium-tin (Nb Sn) embodiment set forth above, otherembodiments manufacturable by the inventive process will be brieflydescribed hereinafter to more fully illustrate the novel concept.

For the binary niobium-aluminum (Nb Al) embodiment, the molten bath 15contains aluminum at about 800C with good infiltration of the aluminuminto the niobium strip being obtained in less than 1 minute, theremainder of the process being the same as above described.

In the ternary niobium-aluminumgermaniumNb (Al,Ge) embodiment, analuminum-germanium eutectic (approximately 53 percent by weight ofgermanium) having a melting point of 424C was used as the infiltrate inthe molten bath 15, with the molten eutectic being maintained at atemperature of about 700C, and with an immersion time of the niobiumstrip in the bath being about 30 seconds. This gave good infiltrationinto the porous niobium, the remainder of the process being carried outas described above.

In the vanadium-gallium (V Ga) embodiment, vanadium powder is rolled, asabove described, to produce a porous vanadium strip which is thereaftersintered in furnace l4 and passed through molten bath 15 where galliumis maintained at a temperature in the range of about 100C, since galliummelts at C, the thus galli um-infiltrated vanadium strip being processedas described above with respect to the niobium-tinembodiment as carriedout by the FIG. 1 apparatus.

As also pointed out above, the amount of reaction of the infiltrate withthe porous metal is related to the time and temperature of the diffusionprocessing step, the times and temperatures being established by seriesof tests.

It has thus been shown that the present invention provides a ductile andeffective superconductive material formed from a porous infiltratedmetal strip having interconnected filaments of a superconducting phaseproduced by a relatively simple and inexpensive process wherein theamount of infiltrated metal converted into the superconducting phase iscontrolled by the amount of deformation, and the time and temperature ofthe diffusion heat treatment. Thus, this invention has greatly advancedthe state of the art.

While a particular apparatus and operation sequence has been illustratedfor producing the novel superconductive material, it is not intended tolimit the method of manufacture to the specifically disclosedoperational sequence of the illustrated apparatus as modifications andchanges will become apparentto those skilled in the art, and it isintended to cover in the appended claims all such modifications andchanges as come within the spirit and scope of the invention.

What we claim is:

1. A method for manufacturing a ductile superconductive materialcontaining interconnecting filaments of a superconducting phase which inthe bulk has a critical field in excess of 100 kilogauss including thesteps of: forming a porous metallic strip having a network ofinterconnecting pores from material selected from the group consistingof niobium and vanadium; sintering the thus formed porous metallicstrip; infiltrating into interconnecting pores of the porous metallicstrip a metallic material selected from the group consisting of tin,aluminum, germanium, antimony, gallium, and mixtures thereof by passingthe metallic strip through a molten bath of the metallic materialwherein the metallic material infiltrates into and substantially fillsthe pores of the metallic strip; reducing the thickness of the thusinfiltrated porous metallic strip up to about 75 percent by passing thestrip through a thickness reducing mechanism; and diffusion heattreating the thus infiltrated metallic strip thereby creatinginterconnecting filament of a superconducting phase of a compound formedby the reaction of the porous metallic strip and the infiltratedmetallic material.

2. The method defined in claim 1, wherein the step of forming themetallic strip is accomplished by com-,

pacting finely powdered metallic material into a strip.

3. The method defined in claim 1, wherein the step of forming themetallic strip is accomplished by compacting hydrides of selectedmetallic material into a strip, the hydrides decomposing to form theporous metallic strip during the sintering step.

4. The method defined in claim 1, wherein the step of diffusion heattreating the thus infiltrated metallic strip is accomplished by passingsame through a dilfusion heat treating furnace wherein at least aportion of the metallic material is reacted with the porous metallicstrip.

5. The method defined in claim 1, additionally including the step ofcollecting the diffusion heat treated metallic strip by rolling same ona collecting apparatus.

'6. The method defined in claim 1, wherein the step of forming theporous metallic strip is accomplished by containing niobium powder of aparticle size in the range of l00 to -400 mesh, and directing the thuscontained niobium powder through a compacting roller apparatus forming acontinuous green" strip of porous niobium.

7. The method defined in claim 6, wherein the step of sintering isaccomplished by passing the porous niobium strip through a sinteringfurnace having an atmosphere selected from the group consisting ofvacuum and inert gas, a temperature in the range of 1,850C to 2,250C,and for a time period of about 3 minutes.

8. The method defined in claim 6, wherein the step of infiltrating theporous niobium strip is accomplished by passing the strip through a bathof molten tin having a temperature range of 500C to- 1000C and for atime period ranging from less than 1 minute to about 3 minutes.

9. The method defined in claim 7, wherein the step of thicknessreduction of the tin-infiltrated strip is accomplished by passing thestrip through a rolling apparatus whereby the thickness of the strip isreduced in the order of up to percent.

10. The method defined in claim 9, wherein the step of diffusion heattreating the tin-infiltrated strip is accomplished by directing thestrip through a diffusion heat-treating furnace having a temperature inthe range of 925C to 1,075C for a time period varying from less than 1minute to several hours depending on the amount of infiltrated tin to beconverted to Nb Sn.

11. The method defined in claim 1, wherein the porous metallic stripconstitutes a porous niobium strip, wherein the step of infiltrating theniobium strip is accomplished by directing the strip through a moltenbath selected from the group consisting of tin, aluminum, antimony,germanium and mixtures thereof, and wherein the diffusion heat treatingof the infiltrated niobium strip creates a superconductive materialcontaining interconnected filaments of a superconducting phase selectedfrom the group consisting of Nb Sn, Nb,,Al, Nb -,(Al,Ge), and Nb Sb.

12. The method defined in claim 1, wherein the porous metallic stripconstitutes a porous vanadium strip, wherein the step of infiltratingthe vanadium strip is accomplished by directing the strip through amolten bath, selected from the group consisting of gallium, aluminum,germanium, and mixtures thereof, and wherein the diffusion heat treatingof the infiltrated vanadium strip creates a superconductivematerialcontaining interconnected filaments of superconducting phase selectedfrom the group consisting of V Ga, V Al, and V (Al,Ge).

13. The method defined in claim 1, wherein the step of diffusion heattreating is accomplished by coiling the thus infiltrated strip, andsubjecting the thus coiled strip to a diffusion heating means.

2. The method defined in claim 1, wherein the step of forming themetallic strip is accomplished by compacting finely powdered metallicmaterial into a strip.
 3. The method defined in claim 1, wherein thestep of forming the metallic strip is accomplished by compactinghydrides of selected metallic material into a strip, the hydridesdecomposing to form the porous metallic strip during the sintering step.4. The method defined in claim 1, wherein the step of diffusion heattreating the thus infiltrated metallic strip is accomplished by passingsame through a diffusion heat treating furnace wherein at least aportion of the metallic material is reacted with the porous metallicstrip.
 5. The method defined in claim 1, additionally including the stepof collecting the diffusion heat treated metallic strip by rolling sameon a collecting apparatus.
 6. The method defined in claim 1, wherein thestep of forming the porous metallic strip is accomplished by containingniobium powder of a particle size in the range of -100 to -400 mesh, anddirecting the thus contained niobium powder through a compacting rollerapparatus forming a continuous ''''green'''' strip of porous niobium. 7.The method defined in claim 6, wherein the step of sintering isaccomplished by passing the poRous niobium strip through a sinteringfurnace having an atmosphere selected from the group consisting ofvacuum and inert gas, a temperature in the range of 1,850*C to 2,250*C,and for a time period of about 3 minutes.
 8. The method defined in claim6, wherein the step of infiltrating the porous niobium strip isaccomplished by passing the strip through a bath of molten tin having atemperature range of 500*C to 1000*C and for a time period ranging fromless than 1 minute to about 3 minutes.
 9. The method defined in claim 7,wherein the step of thickness reduction of the tin-infiltrated strip isaccomplished by passing the strip through a rolling apparatus wherebythe thickness of the strip is reduced in the order of up to 75 percent.10. The method defined in claim 9, wherein the step of diffusion heattreating the tin-infiltrated strip is accomplished by directing thestrip through a diffusion heat treating furnace having a temperature inthe range of 925*C to 1,075*C for a time period varying from less than 1minute to several hours depending on the amount of infiltrated tin to beconverted to Nb3Sn.
 11. The method defined in claim 1, wherein theporous metallic strip constitutes a porous niobium strip, wherein thestep of infiltrating the niobium strip is accomplished by directing thestrip through a molten bath selected from the group consisting of tin,aluminum, antimony, germanium and mixtures thereof, and wherein thediffusion heat treating of the infiltrated niobium strip creates asuperconductive material containing interconnected filaments of asuperconducting phase selected from the group consisting of Nb3Sn,Nb3Al, Nb3(Al,Ge), and Nb3Sb.
 12. The method defined in claim 1, whereinthe porous metallic strip constitutes a porous vanadium strip, whereinthe step of infiltrating the vanadium strip is accomplished by directingthe strip through a molten bath, selected from the group consisting ofgallium, aluminum, germanium, and mixtures thereof, and wherein thediffusion heat treating of the infiltrated vanadium strip creates asuperconductive material containing interconnected filaments ofsuperconducting phase selected from the group consisting of V3Ga, V3Al,and V3(Al,Ge).
 13. The method defined in claim 1, wherein the step ofdiffusion heat treating is accomplished by coiling the thus infiltratedstrip, and subjecting the thus coiled strip to a diffusion heatingmeans.